update async return types (#23340)

The `IAsyncEnumerable` type was missing.

Fixes #17127

docs/csharp/methods.md

@@ -2,7 +2,7 @@
 title: Methods - C# Guide
 description: Overview of methods, method parameters, and method return values
 ms.technology: csharp-fundamentals
-ms.date: 05/21/2018
+ms.date: 03/16/2021
 ms.assetid: 577a8527-1081-4b36-9b9e-0685b6553c6e
 ---
 
@@ -241,7 +241,7 @@ If you mark a method with the [async](language-reference/keywords/async.md) modi
 > [!NOTE]
 > An async method returns to the caller when either it encounters the first awaited object that's not yet complete or it gets to the end of the async method, whichever occurs first.
 
-An async method can have a return type of <xref:System.Threading.Tasks.Task%601>, <xref:System.Threading.Tasks.Task>, or `void`. The `void` return type is used primarily to define event handlers, where a `void` return type is required. An async method that returns `void` can't be awaited, and the caller of a void-returning method can't catch exceptions that the method throws. Starting with C# 7.0, an async method can have [any task-like return type](./whats-new/csharp-7.md#generalized-async-return-types).
+An async method typically has a return type of <xref:System.Threading.Tasks.Task%601>, <xref:System.Threading.Tasks.Task>, <xref:System.Collections.Generic.IAsyncEnumerable%601>or `void`. The `void` return type is used primarily to define event handlers, where a `void` return type is required. An async method that returns `void` can't be awaited, and the caller of a void-returning method can't catch exceptions that the method throws. Starting with C# 7.0, an async method can have [any task-like return type](./whats-new/csharp-7.md#generalized-async-return-types).
 
 In the following example, `DelayAsync` is an async method that has a return statement that returns an integer. Because it is an async method, its method declaration must have a return type of `Task<int>`. Because the return type is `Task<int>`, the evaluation of the `await` expression in `DoSomethingAsync` produces an integer, as the following `int result = await delayTask` statement demonstrates.
 

docs/csharp/programming-guide/classes-and-structs/methods.md

@@ -121,7 +121,7 @@ If you mark a method with the [async](../../language-reference/keywords/async.md
 > [!NOTE]
 > An async method returns to the caller when either it encounters the first awaited object that's not yet complete or it gets to the end of the async method, whichever occurs first.
 
-An async method can have a return type of <xref:System.Threading.Tasks.Task%601>, <xref:System.Threading.Tasks.Task>, or void. The void return type is used primarily to define event handlers, where a void return type is required. An async method that returns void can't be awaited, and the caller of a void-returning method can't catch exceptions that the method throws.
+An async method typically has a return type of <xref:System.Threading.Tasks.Task%601>, <xref:System.Threading.Tasks.Task>, <xref:System.Collections.Generic.IAsyncEnumerable%601>or `void`. The `void` return type is used primarily to define event handlers, where a `void` return type is required. An async method that returns `void` can't be awaited, and the caller of a void-returning method can't catch exceptions that the method throws. Starting with C# 7.0, an async method can have [any task-like return type](./whats-new/csharp-7.md#generalized-async-return-types).
 
 In the following example, `DelayAsync` is an async method that has a return type of <xref:System.Threading.Tasks.Task%601>. `DelayAsync` has a `return` statement that returns an integer. Therefore the method declaration of `DelayAsync` must have a return type of `Task<int>`. Because the return type is `Task<int>`, the evaluation of the `await` expression in `DoSomethingAsync` produces an integer as the following statement demonstrates: `int result = await delayTask`.